The present invention is related to formulations and methods of use of foundry facing and system sand. More particularly a facing sand composition is provided for the retention of casting detail and reduction in casting defects.
The conventional process of sand casting is simply one of forming a hollow mold of a composition of sand and an oil binder and filling it with molten material. A typical casting material is bronze. The binder enables the sand to be molded and to hold its shape as molten metal flows into the cavity defined by the mold. The mold is formed by packing the sand composition about a pattern on a pattern board, removing the pattern to leave a cavity in the formed sand, forming in the sand additional sprues, gates and vents as needed, and pouring the molten material into the cavity. The facing surface of the formed sand dictates the quality or texture of the finished solidified cast product which is ultimately extracted from the sand.
Typically, where it is an objective to produce sand castings having fine surface detail, various difficulties are encountered. For example in the prior art, and as shown in FIG. 1, when using a pattern such as a commemorative plaque having letter or type with partially or tightly facing internal shapes such as xe2x80x9cBxe2x80x9d, xe2x80x9cPxe2x80x9d, xe2x80x9cHxe2x80x9d and xe2x80x9cAxe2x80x9d, sand can remain stuck in the enclosed spaces upon removal of the pattern with the result that the cast letters fill in during casting. Similarly, detail can be lost for patterns having intricate bas relief designs.
Others have partially alleviated this difficulty through the use of a specialized sand composition facing the pattern that contains a greater proportion of finer sand than the remaining bulk of the sand or system sand. System sand typically has a coarser Grain Fineness Number (GFN) of about 70 while finer facing sand is often between a GFN of about 120 and 180. Such fine facing sands can contain clays and are typically less permeable than system sand. Further attempts to increase the cohesive characteristics of the finer sand further reduces the permeability which impedes the escape of burned oil products during hot metal pours and results in xe2x80x9cblowsxe2x80x9d or voids in the resulting casting. The formulation of a fine facing sand is a balance of conflicting characteristics: finer sand providing lower permeability and the binders and other ingredients needed for such compositions further occluding channels between the particles of sand.
Further, tightly packed facing sand is less subject to washing out when the molten metal flows by. Hard compaction is another characteristic which is in conflict with retaining sufficient permeability to permit the release of generated gases and preserve the integrity of the mold.
While it is known to provide specialized facing sand compositions, there is still a need for economical and robust compositions which result in superior performance and surface finishes and which can also be applied successfully using the tools and techniques of the smaller foundries.
Further difficulties are faced when hand tools are used to form the drag and cope of a mold and the methods of applying and compacting the facing sand and the added system sands can additionally and negatively impact the finish of the cast product. Improved methodology are required to assist the facing sand and maintaining a superior surface finish.
A sand casting formulation or composition establishes a balance between providing a fine surface for a superior finished surface and providing maximum permeability for enabling release of gas and maintaining integrity of the mold. Additionally, a novel method of sand application and compaction complements the disclosed composition for maintaining the fine surface finish with a minimum of distortion.
In a preferred embodiment, a facing sand composition is provided which comprises a dry mixture of sand and binder, oil and catalyst. Based on fine sand, a dry mixture is prepared preferably comprising 6% wt/wt binder to fine sand, 8% wt/wt green system sand to fine sand and 12% wt/wt burned system sand to fine sand. Once the oil and catalyst are added to the sand and binder, the proportions being about 8% wt/wt oil to fine sand and 0.5% wt/wt catalyst to fine sand. In a preferred order, the fine sand is mulled with the binder. The green system sand and screened burned system are added and mulled with the fine sand. The oil and catalyst are added. Preferably the catalyst is added last or at least after the binder has been distributed throughout the fine sand fine. The mulled mixture is allowed to rest, is mulled again and allowed to rest again before use.
To ensure accurate reproduction of the pattern""s fine detail, a preferred method of forming the mold is provided. Preferably the cope is prepared in a cope flask. Then the drag, typically bearing the detail, is prepared by first riddling the novel facing sand in a layer over the detail portion of the pattern and compressing it thereon. The facing sand is then backed by riddling a layer of drier system sand over the facing sand to a similar depth as the facing sand and compressing it thereon. A thick layer of system sand is then applied over the entire pattern including over the sides of the pattern and the thick layer of system sand is compacted about the perimeter of the pattern. A further lift of system sand is then applied to span the entire area of the drag flask and is compacted hard about and over the pattern.